The F91 is one of the few Late UC suits that can serve a sniper role by having, you know, an effective long range weapon designed to gun down specific targets in a single, clean shot. Like a sniper rifle, except if the enemy has to be within close proximity then the weapon is going to be more effective since I'm not actually shooting from a safe distance, unseen, like an actual sniper would. I picked it because it was pretty much the only option that'd allow me to keep most of the character intact and require minimal edits from my part. I would've picked something else otherwise, but then most marksman suits are older and therefore shittier specs-wise, and thus would be unable to compete with the given threats or your outlook of "pick something that's high power or get fucked".
Generally speaking there's few sniper units past the OYW because the entirety of the tech development across Universal Century focuses on mobility, since a beam shot will fuck basically anything up in one go anyway. And yes, it has to be a UC line, because most other Universes already have representatives in the game. By my count the only ones not represented sans UC are Age and G. And since G is banned, Age is meh and I'll clearly get fucked over if I pick a GM Sniper or some other reasonable sniper unit...not to mention that it'd require more extensive editing, especially by this point. As for the rest of the team's units:
Raigo has Phase Shift, which renders anything non-beam a non-threat while the F91 has paper armor, and has the distinct advantage of being able to pick and choose equipment for whatever situation pops up and refuel on the go. Having a functional flight pack is already a pretty huge advantage in mobility since, despite what you seem to think, aerial MS battles are rare as fuck in UC, it's why they're obsessed with transforming MS. The Rozen Zulu can't actually fly under gravity at all, for instance. Even Late UC suits are shit at it unless specifically built for it, and while the F91 can go into the air for long enough that it counts as flight, it doesn't mean that it's meant to or that it's actually good at it.
Comparing the X, a high performance unit from an apocalyptic conflict well beyond the scope of the OYW, to the RX-78 is plain nonsense no matter how similar their equipment might be. The only way I can see that matchup is with all the concessions the RX-78 gets for being The Gundam in games and whatnot, but that isn't the case, we're talking powerwank. I mean specs. It'd make more sense to compare the X to something from Wing, if perhaps less extreme about its powerlevel, and even then you seem to be just fine with Arthanus' character making up for his perfectly serviceable unit having basic Gundam equipment by being a super newtype super soldier.
00 Gundams, even the early ones, are on a tier of their own what with having essentially infinite power, effective flight without the need for any kind of specialized equipment and incredibly high power weapons by comparison to other universes, not even going into Trans-Am. Exia is packed with weapons that cut beam shields and barriers, packed with them. It took UC until Crossbone to hit that level of melee, in an era where high speed close range combat is the main battle doctrine, and even then the units that have weapons like those are also cutting edge. The F91 has its guns, which are its main feature and are so unstable the whole thing is a heat-sink and still spazzes out and overheats. I'll take this chance to remind you that Exia also has the humanlike range of motion that bothers you about the mobile trace system.
Sandrock is arguable. Specifically, whether its armor should be able to withstand the accelerated beam shots, which are explicitly designed to deal with beam shields and I-Fields, not extra thick armor made of Plotanium. And even then this is no reason to berate a player for picking out a Mobile Suit he likes.
